1. Field of the Invention
The present invention relates to an electrophotographic printer. In particular, the present invention relates to a wet type electrophotographic printer that uses a developing liquid composed of a liquid toner and a carrier liquid.
Priority is claimed on Japanese Patent Application No. 2009-176742, filed Jul. 29, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
In a wet type electrophotographic printer that uses a developing liquid composed of a liquid toner (hereinafter, also called merely “toner”) and a carrier liquid, a type which transfers toner image formed on an outer periphery surface of a photoconductor drum onto a print surface of a paper via a transfer roller to print image is widely adopted (for example, PCT Japanese Unexamined Patent Application Publication No. 2008-512711). In this type of electrophotographic printer, the toner is fixed to the paper by performing evaporative removal of the carrier liquid, which is absorbed by the paper due to transfer of the toner image onto the paper from the transfer roller (a medium image carrier 301 in PCT Japanese Unexamined Patent Application Publication No. 2008-512711) using a heating roller or the like, for example.
Furthermore, systems are also variously proposed which are configured to pass a web (a belt-like paper) through a plurality of print units including the photoconductor drum, the transfer roller and a backup roller for pinching and pressing the paper between the transfer roller and the backup roller to conduct multi-color printing.
However, as described above, in the case of a system that passes the web through the plurality of units to form multi-color images, there is a problem in that the toner image transferred onto the web print surface in the unit at the upstream side in the web flow direction is inversely transited to the transfer roller of the downstream side unit, which causes a decline in print quality.
In regards to the inverse transition of the toner image from the web print surface to the transfer roller, it is taken into consideration that a bias voltage is applied between the transfer roller and the backup roller to secure a potential difference which makes it easy to generate the transition of the toner from the transfer roller onto the web print surface, whereby the inverse transition is prevented by the bias voltage. The configuration is effective in suppressing the inverse transition of the toner to the transfer roller, but there is a physical limitation in preventing the attachment transition of the toner to the transfer roller due to the nip pressure between the transfer roller and the backup roller. For this reason, it is necessary to develop a technique that can more securely prevent the inverse transition of the toner to the transfer roller to improve the print quality.